High intensity reproducible shock radiation source



Nov. 29, 1966 R. C. ELTON HIGH INTENSITY REPRODUCIBLE SHOCK RADIATION SOURCE Filed Jan. 7, 1964 VACUUM SPECTROGRAPH VACUUM PUMP INVENTOR RAYMOND C. ELTON 7f" (91M AGENT ATTORNEY United States Patent Gfifice 3,289,026 Patented Nov. 29, 1966 3,289,026 HIGH INTENSITY REPRODUCIBLE SHOCK RADIATION SOURCE Raymond C. Elton, Hyattsville, Md., assignor to the United States of America as represented by the Secretary of the Navy Filed Jan. 7, 1964, Ser. No. 336,324 6 Claims. (Cl. 313-231) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention is related to high temperature plasma devices and more partciularly to a device for studying the physical state of partially ionized gases through generation of gaseous shock waves.

Heretofore devices have been used in the production of high temperature plasmas both for use as a light source and as a possible medium in which a controlled thermonuclear reaction may be obtained. high temperature plasmas it is necessary that each generated shock wave be reproducible and operable within a short period of time. Shock waves generated by prior art devices result in less radiation output than required for the desired studies, are not reproducible and shock tubes must be replaced after a few operations. Prior art devices as well as the present invention are described in an article Stark Profile Measurement of the Lyman-a and Lyman-B Lines of Hydrogen by R. C. Elton, Report #417,850 available through the Ofiice of Technical Services, U.S. Department of Commerce.

It is therefore an object of the present invention to provide a device which produces an extremely reproducible output,

Another object is to provide a device which produces copious amounts of radiation for simulating solar and astro-physical conditions.

Still another object is to provide a device which has a high operational repetition rate.

Yet another object is to provide a device which is stable, produces substantially pure gaseous shock waves with plane fronts that approach conditions predicted by ideal shock theory.

The nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawings, in which:

The drawing illustrates a cross-sectional view of the device of this invention including an electrical circuit necessary for the operation of the device.

A T-type electromagnetic shock tube of specific construction is evacuated and a suitable gas of a specific pressure is admitted into the tube.

The sudden deposit of a large amount of energy from a capacitor storage bank results in the rapid generation of a pressure pulse in the arc region which expands down the long shock tube, driving a Mach 12 shock wave in helium before the pressure wave, the reflection of which produces a light source capable of producing a charged particle density of 10 to 10 cm.

Now referring to the drawing there is shown by illustration a T-type shock tube 10 made of quartz in which the cross arm portion 11 is nine centimeters long with an inside diameter of 2.4 centimeters and the length of the quartz tube 12 extending therefrom is of about seventy centimeters in length (75 centimeters to the reflector 45) with a 2.4 centimeter inside diameter. The wall of the shock tube is three milimeters in thickness. Oppositely opposing cylindrical nickel electrodes 13 and 14 are secured in the cross arm 11 with their linear axis perpendicular to the axis of the elongated tubular por- In order to study tion 12 such that the opposing inner ends of the electrodes are separated by 2.5 cm. The electrodes are machined with sections thereon of different diameter such that the diameter of the sections toward the inner end within the cross arm are of decreasing diameter. The opposing ends of the electrodes are rounded and of much smaller diameter than the outer ends of the electrodes. The outer end of each of the electrodes extends out over the end of the cross arm 11 of the shock tube and are separated from the outer ends of the cross arm by suitable O-rings 15. One greased O-ring is placed between the nickle electrode at a section of diameter at least as great as the outside diameter of the cross arm of the shock-tube 11 and the smoothly ground end of this cross arm to act as a vacuum seal. Another section of the electrode is made substantially the same diameter as the inner diameter of the cross arm of the shock tube and is provided with a groove 16 therein within which a split O-ring 15 is placed. This O-ring prevents contamination of the hot gas by the volatile vacuum grease on the seal O-ring and also provides a cushion between the cross arm and the electrodes necessary to prevent breakage of the shock-type cross-arm during operation. The split permits evacuation of the region between the O-rings.

The electrode 14 is provided with holes 17 therein from the outer surface of the sides to the center Where the holes meet with an axially extending passage 18 which is drilled into the electrode. The holes and passage provide a means through which the T-tube may be evacuated by any suitable vacuum system shown in box form and through which a gas under pressure from a gas supply may be admitted into the chamber area within the T-tube.

Metal plate conductors 23 and 24 are connected to electrodes 13 and 14 by semicircular shaped fittings which conform to the outer surface of the cylindrical electrodes. Each of the conductor plates 23 and 24 are connected with a plurality of coaxial cables which feed an electrical current from a capacitor energy storage bank to the electrodes. Such an electrical system is shown by a simple circuit diagram which includes power supplies for charging different capacitors. One circuit triggers the other circuit.

As shown, the electrical circuitry includes suitable direct current power supplies 25, one for the main discharge circuit and one for the triggering circuit. The main discharge circuit includes the power source 25, a 4K ohm current limiting resistor 26 and a capacitor bank 27 made of an assembly of fourteen capacitors of 7.7 mt. each connected in parallel to provide a total capacitance of about 108 mf. having a maximum voltage of 20,000 volts with an energy storage capacity of 22,000 joules. The circuit has a time to peak current of 3 microseconds with a peak current of 1.1 10 amperes. The output circuit from the capacitor includes a 1000 ohm. load resistor 28 in parallel with the spaced electrodes 13 and 14 together in series with a group of spark gap switches 29 connected in parallel which are triggered by a triggering circuit shown. The fourteen capacitors are controlled by enclosed air-gap switches such as described in patent application Serial Number 293,566, filed July 8, 1963, with lone switch per two capacitors. The current is delivered to the collector plate at the shocktube by twenty-one low inductance coaxial cables with a total circuit inductance of 34 nanohenries.

The seven parallel connected air-gap switches are triggered by a 0.1 mt. capacitor 31 operated at 20 kv. and controlled by a single air-gap switch 32 closed by overvoltage. The capacitor 31 is charged by a suitable power supply 25 through one megohm resistors 33 and 34. A ten megohm resistor 35 is placed in the circuit 3 on the load side. The output from the trigger capacitor connects with one electrode of each air gap switch and to a trigger pin which extends through the electrode and is insulated therefrom. A discharge through air-gap switch 32 by overvoltage produces a spark discharge in the main circuit control switch ionizing the air between the electrodes of the main switches to trigger a discharge from the main capacitor bank between the electrodes of the shock tube.

The opposite end of the shock tube is provided with an adapter 41 secured about the end of the tube and sealed against leakage by an O-ring seal 15. The inside diameter of the adapter is the same diameter as the inner diameter of the tube such that there is a continuous wall surface between the tube and the adapter. The adapter provides a means through which the plasma in the tube may be studied by suitable instruments. The adapter mainly provides an optical aperture 42 precisely adjacent to the hot radiating gas for a vacuum spectrograph, not shown for simplification of the drawing. This adjustable optical slit 42 is positioned along the inner Wall of the adapter and sealed against passage of gases from the tube into the vacuum spectrograph except through the optical slit. The adapter is also provided with three quartz windows 43 sealed in the wall thereof such that other instrumentation may be used to study :he gases within the tube.

The outer end of the adapter is sealed by an end :over 44 which is provided with an axial aperture therethrough. A shock reflector 45 has a portion thereof :hat extends through the aperture in the end cover such hat the shock reflector may be adjusted to vary the point at reflection of the shock wave with respect to the point at observation in the adaptor.

In operation of the device for studying the physical itate of partially ionized gases through an electromagietic generation of gaseous shock waves, the vacuum ipectrograph and shock tube are evacuated to a pressure f about 10'- torr, and the device is then filled with lelium gas to a pressure of 40 torr with about 0.03 per- :ent admixture of hydrogen. The main capacitor bank 57 is then charged to 8100 volts. The capacitor bank loes not discharge due to the spacing of the electrodes twitches 29. The trigger circuit capacitor bank 31 is hen charged; at the time that air gap switch 32. is )vervoltaged the capacitor bank 31 discharges across one :lectrode of each of the main air-gap switches and a rigger electrode 30 assemblied in the electrodes. A lischarge between the switch electrode and the trigger :lectrode ionizes the air between the electrodes of the nain-air-gap switches, thus permitting a discharge across he electrodes of the main-air-gap switches 29. The main :apacitor bank 27 then discharges through the system ncluding electrodes 13 and 14 in the shock tube. The lischarge across electrodes 13 and 14 produces an arc vhich in turn, produces a plasma in the area of the arc. the plasma in the vicinity of the are rapidly builds up iressure in this area of the shock tube wherein this plasma :xpands down the length of the shock tube away from he arc, driving a shock wave in front of the pressure Julse. The plasma shock wave is driven through the ielium gas producing ionization of the gas as the plasma hock wave passes down the tube. Ionization of the lelium gas excites the Lyman-a and Lyman-)3 lines of iydrogen which appear immediately behind the shock 'ront. The vacuum spectrograph views this phenomenon hrough the optical slit and records the results. The vaclum, e.c., ultraviolet data may be recorded by use of a cintillator material in combination with a photomultiilier tube. Other optical instruments are used to study he ionized gas through the windows in the adapter. inch measurements include a measure of electron density 1nd temperature as functions of time by use of two mono- :hromators. Shock hydrodynamic conditions are deternined by use of a streak camera.

In coupling the adapter for the vacuum spectrograph to the end of the shock tube, a third electrode at ground potential is established. This establishes an alternative path for the high are current along the tube axis, which can lead to destruction of the shock wave and ultimately damage the vacuum spectrograph slit. Thus, means must be taken to prevent such a current. The vacuum pumping system for the vacuum spectrograph is an integral'part of such a spectrograph and must normally be coupled to ground through the power lines. Therefore the entire spectrograph is electrically floated above ground by incorporating an air-core isolation transformer such as designed for radio tower beacon lighting. Such an isolation transformer provides 50,000 volts of insulation, with 32 mmf. of coupling capacitance while transmitting 4000 watts of power at an efficiency of 94%.

The device is also provided with cooling water which is connected about the vacuum spectrograph by rubber hoses as a further precaution against axial currents to the spectrograph.

The device as presented herein has long life with reproducible results for many operations. With a suitable vacuum system for pumping the system after each operation, the operation may be carried out Within a cycling time of four minutes or less. The electrodes and tube are standard and may be interchangable.

. Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A device for generating highly reproducible gaseous shock waves which comprises;

a quartz shock-tube,

said shock-tube including an open ended elongated tubular portion of about eight times the length of a cross-arm portion.

oppositely disposed nickel electrodes assembled in said cross-arm portion with their longitudinal axis perpendicular to the longitudinal axis of said elongated tubular portion,

said nickel electrodes comprising O-ring cushioning means between themselves and said cross-arm,

and an adapter to be secured to the end of said elongated tubular portion with the inner wall surface thereof in alignmentwith the inner Wall surface of said open ended tubular portion of said shock tube for connecting a vacuum spectrograph thereto at an angle relative to the axis thereof for observing gaseous shock waves produced in said shock-tube.

2. A device for generating reproducible gaseous shock waves which comprises;

a quartz shocktube,

said shock-tube including an open ended elongated tubular portion of about eight times the length of a cross-arm portion,

oppositely disposed nickel electrodes assembled in said cross-arm portion with their longitudinal axis perpendicular to the longitudinal axis of said elongated tubular portion,

said nickel electrodes comprising O-ring cushioning means between themselves and said cross-arm,

one of said electrodes including a passage therein to permit evacuation of said shock-tube,

and an adapter adapted to be secured to the end of said elongated tubular portion with the inner wall surface thereof in alignment with the inner Wall surface of said open ended tubular portion of said shock-tube for connecting a vacuum spectrograph thereto at an angle relative to the axis thereof for observing gaseous shock waves produced in said shock-tube.

3. A device for generating reproducible gaseous shock waves as claimed in claim 2 which includes;

means for evacuating said shock-tube through said passage in said electrode,

means for admitting a gas into said shock-tube through said passage in said electrode, and

bular portion and a cross-arm portion perpendicular thereto,

with the inner surface of said elongated tubular portion to permit optical use of the vacuum spectrograph. 5. A device for generating reproducible gaseous shock waves as claimed in claim 4 which includes;

means for producing an are between said oppositely 5 one of said electrodes includes a passage linearly theredisposed electrodes. through, 4. A device for generating reproducible gaseous shock means for evacuating said shock-tube through said paswaves which comprises; sage in said electrode,

a quartz shock-tube, means for admitting a gas into said shock-tube through said shock-tube including an open ended elongated tu- 10 said passage in said electrode, and

means for producing an arc between said oppositely disposed electrodes.

said elongated portion having a length of about eight times the length of said cross-arm,

oppositely disposed nickel electrodes assembled in said cross-arm coaxial therewith with their ends facing each other in spaced relationship,

said elongated portion and said cross-arm portion having an inside diameter of about the same measure 6. A device for generating reproducible gaseous shock Waves as claimed in claim 5 which includes an adjustable shock reflector within said adapter axially outwardly of said optical slit.

References Cited by the Examiner UNITED STATES PATENTS as said spacing between the ends of said electrodes, said electrodes being separated from the ends and the 2,656,256 10/1953 Yeater 313 7 inner surface of said cross-arm by Oring seals which 29 401011 6/1960 Klob 313-63 serve as a vacuum seal and a cushioning means, 2975332 3/1961 Starr 313 63 an adapter adapted to be secured to the end of said 31005931 10/1961 Dandl 315"-111 elongated tubular portion for connecting a vacuum 3,041,453 6/1962 Daly 250 41-9 spectrograph thereto for observing gaseous shock 3,082,326 3/1963 Arnold 31361 wave produced in said shock-tube,

DAVID J. GALVIN, Primary Examiner. and said adapter including an optical slit in alignment 

1. A DEVICE FOR GENERATING HIGHLY REPRODUCIBLE GASEOUS SHOCK WAVES WHICH COMPRISES; A QUARTZ SHOCK-TUBE, SAID SHOCK-TUBE INCLUDING AN OPEN ENDED ELONGATED TUBULAR PORTION OF ABOUT EIGHT TIMES THE LENGTH OF A CROSS-ARM PORTION. OPPOSITELY DISPOSED NICKEL ELECTRODES ASSEMBLED IN SAID CROSS-ARM PORTION WITH THEIR LONGUTUDINAL AXIS PERPENDICULAR TO THE LONGITUDINAL AXIS OF SAID ELONGATED TUBULAR PORTION, SAID NICKEL ELECTRODES COMPRISING O-RING CUSHIONING MEANS BETWEEN THEMSELVES AND SAID CROSS-ARM, AND AN ADAPTED TO BE SECURED TO THE END OF SAID ELONGATED TUBULAR PORTION WITH THE INNER WALL SURFACE THEREOF IN ALIGNMENT WITH THE INNER WALL SURFACE OF SAID OPEN ENDED TUBULAR PORTION OF SAID SHOCK TUBE FOR CONNECTING A VACUUM SPECTROGRAPH THERETO AT AN ANGLE RELATIVE TO THE AXIS THEREOF FOR OBSERVING GASEOUS SHOCK WAVES PRODUCED IN SAID SHOCK-TUBE. 